1. Field of the Invention
The present invention relates generally to an apparatus and method for expanding the number of antennas in a Multiple Input Multiple Output (MIMO) wireless communication system, and in particular, to an apparatus and method for simultaneously providing a service to users using different numbers of antennas in a Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing (MIMO-OFDM) communication system.
2. Description of the Related Art
The basic issue for communications is how efficiently and reliably to transmit data on channels. Along with the demand for a high-speed communication system capable of processing and transmitting video and wireless data in addition to the traditional voice service, it is essential for future-generation multimedia mobile communication systems now under active study to increase system efficiency using an appropriate channel coding scheme.
Generally, in the wireless channel environment of a mobile communication system, unlike that of a wired channel environment, a transmission signal inevitably experiences loss due to several factors such as multipath interference, shadowing, wave attenuation, time-varying noise, and fading.
The resulting information loss causes severe distortion to the actual transmission signal, degrading the entire system performance. In order to reduce the information loss, depending on the characteristics of the channels many error control techniques are usually adopted to thereby increase system reliability. The basic one use is an error correction code.
In the wireless communication system, multipath fading is relieved by diversity techniques. These techniques are classified into time diversity, frequency diversity, and antenna diversity techniques or schemes.
The antenna diversity scheme uses multiple antennas. This scheme is further branched into receive (Rx) antenna diversity using a plurality of Rx antennas, Tx antenna diversity using a plurality of Tx antennas, and MIMO using a plurality of Tx antennas and a plurality of Rx antennas.
MIMO is a special case of Space-Time Coding (STC) that extends coding in the time domain to the space domain by transmitting a signal encoded in a predetermined coding method through a plurality of Tx antennas, with the aim to achieve a lower error rate.
FIG. 1 is a block diagram of a transmitter in a wireless communication system using a conventional Space-Time Block Coding (STBC). Proposed by Tarokh, the transmitter is comprised of a modulator 100, a Serial-to-Parallel (S/P) converter 102, an STBC encoder 104, and four Tx antennas 106, 108, 110 and 112.
Referring to FIG. 1, the modulator 100 modulates input information data (or coded data) in a predetermined modulation scheme. The modulation scheme can be one of Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), Pulse Amplitude Modulation (PAM), and Phase Shift Keying (PSK).
The S/P converter 102 performs serial to parallel serial conversion modulation symbols received from the modulator 100, s1, s2, s3, s4. The STBC encoder 104 creates eight symbol combinations by STBC-encoding the four modulation symbols, s1, s2, s3, s4 and sequentially transmits them through the four Tx antennas 106 to 112. A coding matrix used to generate the eight symbol combinations is given as Equation 1:
                              G          4                =                  [                                                                      s                  1                                                                              s                  2                                                                              s                  3                                                                              s                  4                                                                                                      -                                      s                    2                                                                                                s                  1                                                                              -                                      s                    4                                                                                                s                  3                                                                                                      -                                      s                    3                                                                                                s                  4                                                                              s                  1                                                                              -                                      s                    2                                                                                                                        -                                      s                    4                                                                                                -                                      s                    3                                                                                                s                  2                                                                              s                  1                                                                                                      s                  1                  *                                                                              s                  2                  *                                                                              s                  3                  *                                                                              s                  4                  *                                                                                                      -                                      s                    2                    *                                                                                                s                  1                  *                                                                              -                                      s                    4                    *                                                                                                s                  3                  *                                                                                                      -                                      s                    3                    *                                                                                                s                  4                  *                                                                              s                  1                  *                                                                              -                                      s                    2                    *                                                                                                                        -                                      s                    4                    *                                                                                                -                                      s                    3                    *                                                                                                s                  2                  *                                                                              s                  1                  *                                                              ]                                    (        1        )            where G4 denotes the coding matrix for symbols transmitted through the four Tx antennas 106 to 112 and s1, s2, s3, s4 denote the input four symbols to be transmitted. The columns of the coding matrix correspond to the Tx antennas and the rows correspond to time intervals in which the four symbols are transmitted. Thus, the four symbols are transmitted through the four Tx antennas for eight time intervals.
Specifically, for a first time interval, s1 is transmitted through the first Tx antenna 106, s2 through the second Tx antenna 108, s3 through the third Tx antenna 110, and s4 through the fourth Tx antenna 112. In this manner, −s4*, −s3*, s2*, s1* are transmitted through the first to fourth Tx antennas 106 to 112, respectively for an eighth time interval. That is, the STBC encoder 104 sequentially provides the symbols of an ith column in the coding matrix to an ith Tx antenna.
As described above, the STBC encoder 104 generates the eight symbol sequences using the input four symbols and their conjugates and negatives and transmits them through the four Tx antennas 106 to 112 for eight time intervals. Since the symbol sequences for the respective Tx antennas, that is, the columns of the coding matrix are mutually orthogonal, as high a diversity gain as a diversity order is achieved.
FIG. 2 is a block diagram of a receiver in the wireless communication system using the conventional STBC scheme. The receiver is the counterpart of the transmitter illustrated in FIG. 1.
The receiver is comprised of a plurality of Rx antennas 200 to 202, a channel estimator 204, a signal combiner 206, a detector 208, a Parallel-to-Serial (P/S) converter 210, and a demodulator 212.
Referring to FIG. 2, first to Pth Rx antennas 200 to 202 provide signals received from the four Tx antennas of the transmitter illustrated in FIG. 1 to the channel estimator 204 and the signal combiner 206.
The channel estimator 204 estimates channel coefficients representing channel gains from the Tx antennas 106 to 112 to the Rx antennas 200 to 202 using the signals received from the first to Pth Rx antennas 200 to 202.
The signal combiner 206 combines the signals received from the first to Pth Rx antennas 200 to 202 with the channel coefficients in a predetermined method.
The detector 208 generates hypothesis symbols using the combined symbols and the channel coefficients, calculates decision statistics for all possible transmitted symbols from the transmitter using the hypothesis symbols, and detects the actual transmitted symbols through threshold detection.
The P/S converter 210 performs parallel to serial conversion of the symbols received from the detector 208. The demodulator 212 demodulates the serial symbol sequence in a predetermined demodulation method, thereby recovering the original information bits.
As described above, the Tarokh scheme being an expansion of an Alamouti STBC technique offers the benefit of achieving a maximum diversity order using an STBC code in the form of a matrix with orthogonal columns. However, because four complex symbols are transmitted for eight time intervals, the Tarokh STBC scheme provides a rate of ½.
To achieve a full rate in a MIMO system that transmits complex signals through three or more Tx antennas, the Giannakis group presented a Full-Diversity, Full-Rate (FDFR) STBC for four Tx antennas using constellation rotation over a complex field.
FIG. 3 is a block diagram of a transmitter in a wireless communication system using the conventional Giannakis STBC scheme. As illustrated in FIG. 3, the transmitter includes a modulator 300, a pre-coder 302, a space-time mapper 304, and a plurality of Tx antennas 306, 308, 310 and 312.
Referring to FIG. 3, the modulator 300 modulates input information data (or coded data) in a predetermined modulation scheme such as BPSK, QPSK, QAM, PAM or PSK.
The pre-coder 302 pre-codes Nt modulation symbols received from the modulator 300, d1, d2, d3, d4 such that signal rotation occurs in a signal space, and outputs the resulting Nt symbols. For notational simplicity, four Tx antennas are assumed. Let a sequence of four modulation symbols from the modulator 300 be denoted by d. The pre-coder 302 generates a complex vector r by computing the modulation symbol sequence, d using the following Equation 2:
                    r        =                              Θ            ⁢                                                  ⁢            d                    =                                                    [                                                                            1                                                                                      α                        0                        1                                                                                                            α                        0                        2                                                                                                            α                        0                        3                                                                                                                        1                                                                                      α                        1                        1                                                                                                            α                        1                        2                                                                                                            α                        1                        3                                                                                                                        1                                                                                      α                        2                        1                                                                                                            α                        2                        2                                                                                                            α                        2                        3                                                                                                                        1                                                                                      α                        3                        1                                                                                                            α                        3                        2                                                                                                            α                        3                        3                                                                                            ]                            ⁡                              [                                                                                                    d                        1                                                                                                                                                d                        2                                                                                                                                                d                        3                                                                                                                                                d                        4                                                                                            ]                                      =                          [                                                                                          r                      1                                                                                                                                  r                      2                                                                                                                                  r                      3                                                                                                                                  r                      4                                                                                  ]                                                          (        2        )            where Θ denotes a pre-coding matrix. The Giannakis group uses a Vandermonde matrix being a unitary one as the pre-coding matrix. In the pre-coding matrix, αi is given as Equation 3:αi=exp(j2π(i+¼)/4), i=0,1,2,3  (3)
The Giannakis STBC scheme uses four Tx antennas and is easily extended to more than four Tx antennas, as well. The space-time mapper 304 STBC-encodes the pre-coded symbols in the following Equation 4:
                    S        =                  [                                                                      r                  1                                                            0                                            0                                            0                                                                    0                                                              r                  2                                                            0                                            0                                                                    0                                            0                                                              r                  3                                                            0                                                                    0                                            0                                            0                                                              r                  4                                                              ]                                    (        4        )            where S is a coding matrix for symbols to be transmitted through the four Tx antennas 306 to 312. The columns of the coding matrix correspond to the Tx antennas and the rows correspond to time intervals required to transmit the four symbols. That is, the four symbols are transmitted through the four Tx antennas for the four time intervals.
Upon receipt of the four symbols on a radio channel for the four time intervals, a receiver (not shown) recovers the modulation symbol sequence, d by Maximum Likelihood (ML) decoding.
As described above, Spatial Diversity (SD) achieves transmit diversity by transmitting the same data through multiple antennas. However, the diversity order increases with the number of transmit antennas but decreases a gain increase rate. In other words, as the number of antennas increases, the diversity order is saturated rather than linearly increasing.
In contrast, Spatial Multiplexing (SM) offers the benefit of high-speed data transmission without increasing the bandwidth of the system by transmitting different data at the same time using multiple antennas in the transmitter and the receiver.
FIG. 4 is a block diagram of a wireless communication system using a conventional SM scheme. As illustrated, a transmitter is comprised of a modulator 400, an S/P converter 402, and four Tx antennas 404, 406, 408 and 410. A receiver is comprised of four Rx antennas 414, 416, 418 and 420 and a reception part 412.
Referring to FIG. 4, the modulator 400 modulates input information data (or coded data) in a predetermined modulation scheme.
The S/P converter 302 spatially multiplexes four modulation symbols s1, s2, s3, s4 received from the modulator 400 as Equation 5:
                    S        =                  [                                                                      s                  1                                                                                                      s                  2                                                                                                      s                  3                                                                                                      s                  4                                                              ]                                    (        5        )            where the columns correspond to the Tx antennas and the rows correspond to time intervals required for transmitting the four transmission symbols. Since four symbols are transmitted for one time interval, the data rate is 4.
Meanwhile, the reception part 412 of the receiver estimates the transmitted symbols s1, s2, s3, s4 from signals received through the four Rx antennas 414, 416, 418 and 420.
As described above, there are a number of multiple antenna schemes (or MIMO schemes). Hence, an optimum multiple antenna scheme needs to be selected according to users' demands or situations. Since the wireless communication system is rapidly being developed to provide high-speed, high-quality service to users, it is expected that the number of Tx antennas will increase for better link performance. In this case, required MIMO schemes and required numbers of Tx antennas may differ for different users. In other words, users may request a throughput increase or a data reliability increase under circumstances. For example, a voice user may request a low error rate rather than a rate increase, whereas a data user may request a high rate.
Therefore, it is critical to support a multiple antenna scheme according to a user's situation in future communication systems.
Especially, there exists a need for a method of effectively providing a service to a plurality of users each adopting a different MIMO scheme and a different number of antennas without modifying the existing frame structure, when the number of Tx antennas increases in legacy systems.